In prior aircraft power generating systems, the output power produced by a brushless, synchronous generator is supplied to an electrical power converter which converts the variable-frequency AC output of the generator into fixed frequency power. The power converter, as well as other devices used in the power generating system, such as an exciter current regulator, require control or operating power. In the past, this has been supplied by a relatively small auxiliary permanent magnet generator (PMG) which typically formed a part of the brushless, synchronous generator.
Synchronous generators of the foregoing type have successfully been used in many aerospace and aircraft installations. In such installations, however, system size and weight must be kept to a minimum. Recently, advances in magnetic materials have resulted in the design of high power permanent magnet generators which are capable of the high power levels required in aerospace and aircraft installations and which are small and light in weight. Axial gap PMG's, i.e. those where the magnetic flux developed by the permanent magnet is directed in an axial direction, have a high power density and thus can be made smaller and lighter than conventional radial gap PMG's of similar load capacity. Therefore, an overall savings in size and weight can be realized. However, the need for control power for the power converter connected to the PMG remains, and hence some provision must be made to supply this power requirement.
Hawsey, et al., U.S. Pat. No. 4,996,457 discloses a high speed permanent magnet axial gap alternator. First and second permanent magnet structures are carried by a rotor and are disposed axially side-by-side and are separated by a material providing magnetic isolation therebetween. First and second stators carrying first and second windings, respectively, are disposed adjacent the first and second magnets. Rotation of the rotor causes flux lines developed by the magnets to cut the windings in the stators, thereby inducing voltages therein. The magnetic isolation between the rotor magnets prevents one load connected to one of the windings from affecting another load connected to the other winding.
Nichols, et al., U.S. Pat. No. 3,215,876 discloses a generator unit including first and second permanent magnet rotor structures disposed on opposite sides of a stator and an armature winding wound on the stator wherein the armature winding is disposed in a useful path and in a leakage flux path. All of the flux developed by the magnetic structures is thus available for developing power for a load connected to the generator output.
Richter, U.S. Pat. No. 4,371,801 and Pullen, et al., U.S. Pat. No. 5,021,698 disclose axial gap permanent magnetic generators having multiple rotors and multiple stators. In the case of the Richter device, the stator windings are connected in series and the placement of a portion of the stator windings may be varied relative to the other stator windings to achieve output voltage regulation.
Other types of axial gap dual permanent magnetic generators (DPMG's) are disclosed in Lynch, et al., U.S. Pat. No. 5,245,238, entitled "Axial Gap Dual Permanent Magnet Generator" and assigned to the assignee of the instant application, the disclosure of which is hereby incorporated by reference herein. This application discloses DPMG's having two relatively movable rotors each carrying a plurality of permanent magnets disposed adjacent one or two fixed stator windings. The positions of the rotor magnets relative to one another is adjusted to vary the output voltage of the generator.